The present invention relates to a brake pressure control device for automotive vehicles for incorporation between a brake master cylinder and a rear wheel brake cylinder to control the ratio of brake fluid pressure being applied to the front and rear wheel brake cylinders in response to the attainment of a predetermined rate of deceleration of the vehicle, and more particularly to a brake pressure control device in which an inertia-controlled valve element such as a metallic ball cooperates with a valve seat in a valve chamber to control the ratio of brake fluid pressure being applied to the front and rear wheel brake cylinders and in which the initial space between the valve element and the valve seat changes in response to increase or decrease of the master cylinder pressure.
As is illustrated in FIG. 1, a conventional brake pressure control device 10 of this kind comprises a housing assembly 11 provided with an inlet port 11a for connection to a brake master cylinder and an outlet port 11b for connection to a rear wheel brake cylinder, a pressure responsive piston 15 axially slidably disposed within a stepped bore 11c in housing assembly 11 to form fluid chambers 12, 13, 17 and an air chamber 14, a compression coil spring 16 disposed within the fluid chamber 13 to bias the piston 15 rightwards in the figure, an inertia-controlled valve element 18 in the form of a metallic ball disposed within the fluid chamber 17 in open communication with the fluid chamber 13 via a passage 11d, and a plunger 20 carried by a spring 19 to receive the valve element 18 thereon and to be displaced by the pressure in fluid chamber 17. The housing assembly 11 is fixedly mounted on a vehicle body structure at an angle 0 relatively to a horizontal line in such a manner that the axis of the axial bore 11c is located in a fore-and-aft direction of the vehicle. Under inoperative condition of the brake system, the piston 15 and plunger 20 are held by respective loads of springs 16 and 19 in their initial positions shown in the figure, and the inlet port 11a is in open communication with the outlet port 11b via fluid chamber 13, passage 11d, fluid chamber 17 and a space between the valve element 18 and a valve seat 11e in fluid chamber 17.
In braking operation, the pressure PW applied to the rear wheel brake cylinder increases at the same rate as that of the master cylinder pressure PM. (see FIG. 2) When the master cylinder pressure PM reaches a level A on the graph of FIG. 2, the piston 15 displaces against spring 16, and a projection 15a of piston 15 retracts to permit engagement of the valve element 18 with the valve seat 11e. If the vehicle is being applied with a light load, the deceleration of the vehicle will exceed a predetermined value in response to further increase of the master cylinder pressure PM to a level B on the graph of FIG. 2. In such a condition, the valve element 18 starts to engage the valve seat 11e so as to interrupt the fluid communication between inlet and outlet ports 11a and 11b, while the piston 15 abuts against the end wall of air chamber 14. Further increase of the rear wheel brake cylinder pressure PW from the level B to a level C will be effected until engagement of the valve element 18 with the valve seat 11e is effected. During further increase of the master cylinder pressure PM up to a level D, the rear wheel brake cylinder pressure PW will be maintained at the level C. When master cylinder pressure PM exceeds the level D, the piston 15 will displace rightwards to disengage the valve element 18 from the valve seat 11e so as to permit further increase of the rear wheel brake cylinder pressure PW under control of the piston 15.
If the vehicle is being applied with a heavy load in the braking operation, the deceleration of the vehicle will exceed the predetermined value when the master cylinder pressure PM has increased to a level F. During such increase of the master cylinder pressure PM to level F, the plunger 20 will retract against spring 19, and the valve element 18 will roll rearwardly to increase the space between the valve element 18 and the valve seat 11e. As a result, a time necessary for engagement of the valve element 18 with the valve seat 11e becomes longer than that under the light loaded condition of the vehicle. Consequently, the pressure increase from level F to a level G becomes larger than that from level B to level C on the graph of FIG. 2. Thereafter, the rear wheel brake cylinder pressure PW will be controlled substantially as same as that under the light loaded condition of the vehicle.
In operation of such conventional devices as described above, there occurs a time delay when the valve element 18 rolls rearwardly by its inertia in response to retraction of the plunger 20 against spring 19. The valve element 18 tends to rest in place by its gravity when the plunger 20 is retracted. Rearward rolling of the valve element 18 towards the plunger 20 is delayed by the difference in pressure acting on the valve element 18, and the space between the valve element 18 and the valve seat 11e is reduced due to forward movement of the valve element 18 caused by the difference in pressure. For these reasons, when the brake pedal is rapidly depressed, the valve element 18 does not retract in response to retraction of the plunger 20 and tends to roll forwardly prior to engagement with the piston 20. This results in unstable control of the master cylinder pressure.